Node undici: undici's BalancedPool constructor passes its entire options object through an internal deep-clone that serializes and reparses the value as JSON. (CVE-2026-84961)
Description
Node.js undici library's BalancedPool constructor improperly handles TLS options by deep cloning its options via JSON serialization, which drops function-valued TLS options such as custom checkServerIdentity callbacks. This leads to TLS certificate validation bypass when using BalancedPool with such custom callbacks, allowing acceptance of TLS peers that should be rejected. Other undici components like Client, Pool, and Agent are not affected. The issue is fixed in versions 7.29.1 and 8.10.2.
CVSS v3.1
Score 9.1critical
Affected software
pkg:deb/ubuntu/node-undici?arch=source&distro=noblepkg:deb/ubuntu/node-undici?arch=source&distro=resolutepkg:deb/ubuntu/node-undici?arch=source&distro=stonkingRun on your own infrastructure? Check whether these packages are installed with threat-finder — our free open-source scanner.
Weaknesses
AI-Powered Analysis
Machine-generated threat intelligence
Technical Analysis
The vulnerability in node-undici's BalancedPool component arises because its constructor deep clones the entire options object by serializing and reparsing it as JSON. This process removes function-valued TLS options, including custom checkServerIdentity callbacks used for certificate validation. As a result, TLS certificate validation can be bypassed when BalancedPool is used with such custom callbacks, potentially allowing acceptance of malicious TLS peers. This flaw does not affect other undici components such as Client, Pool, or Agent. The vulnerability is tracked as CVE-2026-84961 and has a CVSS 3.1 base score of 9.1, indicating high severity. Fixes are available in versions 7.29.1 and 8.10.2.
Potential Impact
Successful exploitation allows attackers to bypass TLS certificate validation when using the BalancedPool component with custom TLS options, potentially enabling man-in-the-middle attacks by accepting TLS peers that should be rejected. This compromises the confidentiality and integrity of TLS connections established via BalancedPool in affected versions.
Mitigation Recommendations
Upgrade node-undici to version 7.29.1 or later if using the 7.x branch, or to version 8.10.2 or later if using the 8.x branch. These versions contain the official fix for this vulnerability. No other mitigation is required as the issue is resolved in these releases.
Technical Details
- Gcve Source
- db.gcve.eu
- Osv Id
- UBUNTU-CVE-2026-84961
- Osv Schema Version
- 1.7.0
- Ecosystems
- ["Ubuntu:24.04:LTS","Ubuntu:26.04:LTS"]
- Cvss Version
- 3.1
Threat ID: 6a9ee1b1acd9273b49e69d0d
Added to database: 09/07/2026, 16:09:21 UTC
Last enriched: 10/18/2026, 02:25:15 UTC
Last updated: 10/23/2026, 01:15:27 UTC
Views: 86
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